Studies involving gene expression
profiling and mass spectrometry suggest that neuropathic pain is associated
with a strong activation of certain neuronal genes, as well as genes associated
with immune cell responses including microglial activation. Studies published
in Molecular Pain in 2014 enabled us
to identify the genes whose expression changes after sciatic nerve injury and
after multiple intraperitoneal minocycline administrations in this model.

DNA
microarray methods showed changes in the activity of the enzymes and products
of kynurenine pathway in the lumbar spinal cord in neuropathic pain model. The
next step of our study was to select gene expression changes induced by chronic
constriction injury (CCI) of the sciatic nerve that are modulated by minocycline,
which can effectively diminish neuropathic pain in animal studies.One of these genes - KMO (kynurenine
3-monooxygenase - kynurenine pathway enzyme) has become the subject of further
research, and its changes on the mRNA level after the injury were confirmed by qRT-PCR and western blot, and additionally they were
demonstrated in dorsal root ganglia (DRG).

Our
primary glial cells culture indicate that minocycline reverses LPS-induced
microglial activation and decreases Kmo mRNA expression in primary microglial
cells.Our results [1] show that
both Kmo inhibitors (Ro61-6048 and JM6) diminish the symptoms of neuropathic
pain after sciatic nerve injury and enhance the effectiveness of morphine in a
neuropathic pain model. Our biochemical studies show that seven days after
sciatic nerve injury, repeated Ro61-6048 administrations reduce the
microglia/macrophages activation in the spinal cord and DRG. Furthermore, the
mRNA and protein levels of pronociceptive factors in the spinal cord (i.e.,
IL-6 and NOS2) and DRG (i.e., IL-1beta, IL-6 and NOS2) were reduced after
Ro61-6048 treatment.As our data show, the modulation
of Kmo enzyme can significantly influence nociceptive processes (Scheme 1), exhibit neuroprotective
effects, and increase opioid efficacy. These results will also allow us to
recognize a wide spectrum of tryptophan metabolites belonging to kynurenine
pathway, what is the subject of grant SONATA 9 2015/17/D/NZ4/02284.